Petroleum residua consist of an ordered continuum of solvated polar materials usually referred to as asphaltenes dispersed in a lower polarity solvent phase held together by intermediate polarity materials usually referred to as resins.
Refinery processing of petroleum residua such as heavy oils, shale oils, coal tars, tar sand bitumen, asphalts, or the like, to obtain lighter distillate fuels require heating for distillation, hydrogen addition, or carbon rejection (coking). Typically, refinery processing does not include the addition of solvents, non-solvents, precipitation agents, flocculation agents, or the like to petroleum residua during heating. Efficiency of such refinery processing can be limited by the formation of insoluble carbon-rich materials such as coke as the amount of resins are depleted. Heat exchangers and other refinery units must be shut down for mechanical coke removal, resulting in a significant loss of output and revenue. When a petroleum residuum is heated above the temperature at which pyrolysis occurs (greater than about 340° C., 650° F.), there is typically an induction period before coke formation begins. To avoid fouling of refinery equipment, refiners often stop heating a petroleum residuum based upon arbitrary criteria before coke formation begins. Because arbitrary criteria are used thermal treatment is stopped sooner than is necessary, resulting in less than maximum product yield.
The coking indexes disclosed in U.S. patent application Ser. Nos. 60/138,846; 10/009,863, now U.S. Pat. No. 6,373,921, and International Patent Application No. PCT US00/15950, each hereby incorporated by reference herein, can be used by refiners to assess immediacy of petroleum residua to the threshold of coke production. U.S. Pat. No. 6,773,921 discusses various ways to analyze coke formation such as but not limited to coking indexes, filtration, asphaltene flocculation titration, solubility characteristics of asphaltenes, values of a weight percent of a cyclohexane soluble portion of heptane asphaltenes, relative viscosity, KS values, KF values, and K values. As such, refiners can thermally treat petroleum residua to the threshold, but not beyond the point at which coke formation begins to form when petroleum residua materials are heated at pyrolysis temperatures. See also, Schabron, J. F., A. T. Pauli, and J. F. Rovani, Jr., “Molecular Weight/Polarity Map for Residua Pyrolysis”, fuel, 80 (4), 529–537 (2001); Schabron, J. F., A. T. Pauli, and J. F. Rovani, Jr., “Non-Pyrolytic Heat Induced Deposition from Heavy Oils”, Fuel, 80 (7), 919–928 (2001); and Schabron, J. F., A. T. Pauli, J. F. Rovani, Jr., and F. P. Miknis, “Predicting Coke Formation Tendencies”, Fuel, 80 (11), 1435–1446 (2001), each hereby incorporated by reference herein.
The development of such coking indexes provide universal predictors provide a solution to the long standing, but unresolved problem of determining immediacy of a petroleum residua to coke formation during petroleum residua refining. As such, these coking indexes have great potential value in improving the efficiency of thermal treatment of petroleum residua during distillation or other processes.
Prior to the instant invention, there were unresolved limitations with respect to the use of these coking indexes. First, the determination of the various coking indexes was by either titration or by solubility measurements performed in a laboratory. See for example, U.S. patent application Ser. No. 60/266,555 and International Patent Application No. PCT US02/03983, each hereby incorporated by reference herein. Secondly, because these coking index determinations are made off line an amount of time elapse prior to obtaining the results which may not then accurately reflect the then existing properties of the petroleum residuum which has continued along in the thermally treatment process. As such, to avoid formation of coke, or other solid material, from petroleum residue undergoing pyrolytic thermal treatment, thermal treatment may have been stopped, or the manner of thermal treatment may have been altered to compensate for the delay in obtaining coking indexing determinations.
When distillation is stopped sooner than is necessary to avoid coke formation, it can result in less than maximal product yield from the hydrocarbon material. In 1997, for example, the average United States atmospheric and vacuum distillation refinery capacity was about 23 million barrels per day as disclosed by the Department of Energy, OIT Report, p. 5, (1998), hereby incorporated by reference. Solvent deasphalting capacity was about 0.3 million barrels per day. About 1.8 million barrels per day of heavy end feedstocks produced in 1997 from atmospheric and vacuum distillation columns and solvent-deasphalting units were input to thermal cracking and coking operations. This represents about 10% of the crude run. Id. at p. 49. An additional 6.5 million barrels per day went into catalytic cracking and hydro treating units. Based on the total of 1.8 million barrels of total heavy ends minus about 0.3 million from solvent deasphalting, about 1.5 million barrels of heavy ends per day of thermal cracking and coking feed are produced from distillation operations. Assuming a one percent increase in United States distillate output because of efficiency improvements, an increase of about 15,000 average barrels per day of distillate and a corresponding reduction of heavy ends would result. Efficiency increases well above 1% could be possible if the immediacy to coking for a petroleum residua could be determined on-line to avoid interruption of thermal treatment prematurely.
Moreover, coking operations use about 166,000–258,000 Btu per barrel of petroleum residua feed. Department of Energy, OIT Report, pp. 62–63, (1998). Hydrotreater energy use is comparable, and a similar consideration may apply. Since most of the energy used is to initially heat all of the petroleum residua feed material for distillation, there may be only minimal extra heat required to obtain a 1% improvement of distillate output at a particular temperature. For each 1% decrease in hydrocarbon material feed, there would be a potential savings of about 2.5–3.9 billion Btu with respect to petroleum residua feed that do not need to be heated, since they will have been recovered in an optimized distillate stream.
Another significant problem with conventional technology for the evaluation and processing of petroleum residua may be high emissions. An energy savings of about 2.5–3.9 billion Btu per day, as discussed above, can result in a corresponding lowering of emissions from fuel that is not burned in processing operations. For example, residual fuel used as the heat source produces about 174 pounds of carbon dioxide per million Btu generated Department of Energy, OIT Report, pp. 27, (1998). Thus, in the U.S., the reduction in carbon dioxide emissions for each 1% industry-wide efficiency improvement may be about 218–679 tons per day!
Another significant problem with conventional technology for the evaluation and processing of petroleum residua may be financial losses. The disruption of petroleum residua processing from fouling due to deposition of solid material, such as coke, is pervasive throughout the industry. The financial losses due to unscheduled downtime events as a result of non-pyrolytic, or of pyrolytic, deposition of carbon rich materials such as coke, may be difficult to quantify, but they are important.
Another significant problem with conventional technology for the evaluation and processing of petroleum residua may be that the liquid products of distillation may be of lower quality. Interrupting the distillation process, or proceeding with the distillation process in steps or stages, to avoid deposition of carbon rich materials or coke may allow for contamination of the liquid distillates.
The invention addresses the various problems associated with determination of coking indexes in the laboratory by providing apparatus and methods that can provide continuous on-line monitoring of petroleum residua processing to avoid coke formation.